1. Field of the Invention
The present invention relates to a liquid crystal display device for displaying character information and image information using a TN (twisted nematic) liquid crystal cell or an STN (super twisted nematic) liquid crystal cell and, more specifically, to a reflection-type liquid crystal display device and a transflective liquid crystal display device.
2. Description of the Related Art
As a conventional liquid crystal display device for displaying character information and image information by a liquid crystal display, there is, for example, one described in JP, 54-153066, A.
When character information and image information are displayed using a liquid crystal, a liquid crystal display device, as described in the aforementioned Laid-open, is used, in which absorption-type polarizing films each having a transmission axis and an absorption axis are disposed on the upper and lower sides, respectively, of a liquid crystal cell made by sealing the liquid crystal between two transparent substrates so that the transmission axes thereof are directed at predetermined angles.
When an electric field is applied to the liquid crystal display device, an alignment direction of the liquid crystal having permittivity anisotropy changes, and thus its optical characteristics change. Through the use of the optical anisotropy of the liquid crystal, a predetermined display is performed.
As a typical conventional liquid crystal display device described above, there is a reflection-type liquid crystal display device used with a twisted nematic (TN) liquid crystal sealed in a liquid crystal cell.
In this case, a linearly polarized light which has been made incident from the visible side and has passed through a first absorption-type polarizing film, its polarization direction being controlled by optical activity of the TN liquid crystal when passing through the liquid crystal cell, passes through a second absorption-type polarizing film or is absorbed there.
Therefore, the second absorption-type polarizing film is disposed so that when the TN liquid crystal has optical activity without application of voltage, a linearly polarized light which has been optically rotated by the TN liquid crystal passes through the second absorption-type polarizing film, and a white plate is disposed behind it, whereby the light reflected by the white plate is emitted out to the visible side via a reverse route, presenting a white display.
Meanwhile, the TN liquid crystal in a portion where a voltage is applied thereto loses the optical activity, whereby a linearly polarized light which has passed through the first absorption-type polarizing film is made incident on the second absorption-type polarizing film as it is and absorbed, and thus the light is not emitted out to the visible side, resulting in a black display. Accordingly, this case is a normally-white mode, in which character information and image information are displayed in black against a white background.
However, a display of character information and image information in black against the white background as described above has nothing especially novel appearance in visual design, offering neither excellent fashion nor particular interest.
Therefore, a liquid crystal display device is desired which is capable of providing variation in display state of character information and image information and additionally performing a display distinctive in design.
Moreover, in order to increase the amount of information such as character information and image information capable of displaying, it is necessary to use not a liquid crystal display device using the TN liquid crystal but a liquid crystal display device using a super twisted nematic (STN) liquid crystal which has highly sharp responsivity in optical characteristic change by application of voltage and is capable of a high-split display because a large margin can be obtained.
Since a liquid crystal display device using the STN liquid crystal is in a display mode using a birefringence anisotropy of the liquid crystal, a display color of the background to which no voltage is applied is colored due to an elliptically polarized light even if using a normally-white mode which is excellent in brightness for a monochrome display.
Consequently, the display color of the background is not white but a color close to yellowish green, and the display color of the black display portion to which voltage is applied becomes bluish black. Thus, even the reflection-type liquid crystal display device using the STN liquid crystal with a large display capacity and excellent contrast provides no change in design, resulting in a plain information display with little charm.
Therefore, it is desired to produce an information display in an achromatic color without coloring, new in design and fashionable, through the reflection-type liquid crystal display device using the STN liquid crystal.
This invention is made in view of the status quo, and its object is to provide a liquid crystal display device capable of providing variation in display of character information and image information, variation in design, and excellent fashion, and further increasing the capacity of information display.
To attain the above-described objects, a liquid crystal display device according to the present invention comprises: a TN liquid crystal cell constituted by sandwiching a nematic liquid crystal which is aligned at a twist angle of about 90xc2x0 between a first and a second substrate each having transparent electrodes; an absorption-type polarizing film provided on the outside, which is the visible side of the TN liquid crystal cell, of the second substrate; and a reflection-type polarizing film, a scattering layer, and a mirror reflector provided in order on the outside, which is the opposite side to the visible side of the TN liquid crystal cell, of the first substrate, thereby constituting a reflection-type liquid crystal display device.
With this structure, a reflection display of information of character and image can be performed in an achromatic metallic tone with high contrast within a bright diffused white background to improve the visual design and fashion of the liquid crystal display device.
A liquid crystal display device according to the present invention may be constituted by comprising: an STN liquid crystal cell constituted by sandwiching a nematic liquid crystal which is aligned at a twist angle from 180xc2x0 to 270xc2x0 between a first and a second transparent substrate each having transparent electrodes; a retardation film and an absorption-type polarizing film provided in order on the outside, which is the visible side of the STN liquid crystal cell, of the second substrate; and a reflection-type polarizing film, a scattering layer, and a mirror reflector provided in order on the outside, which is the opposite side to the visible side of the STN liquid crystal cell, of the first substrate. This constitutes a reflection-type liquid crystal display device having a great display capacity in addition to the aforesaid effects, and also prevents coloring of a display.
The mirror reflector in each of these liquid crystal display devices is a mirror transflective reflector, and a backlight is provided on the outside of the mirror transflective reflector, thereby also constituting a transflective liquid crystal display device capable of reflection-type display by external light and transmission-type display by light emitted by the backlight.
In the liquid crystal display device including the STN liquid crystal cell and the retardation film, a Z-type retardation film which satisfies conditions of nx greater than nz greater than ny, where nx is a refractive index in a stretching direction of the retardation film, ny is a refractive index in a planar direction orthogonal to the stretching direction, and nz is a refractive index in a thickness direction, is used as the aforesaid retardation film, whereby viewing angle characteristics are improved to use also incident light from the surroundings efficiently, resulting in a brighter reflection display.
Further, it is preferable that the retardation film is a twisted retardation film, and a difference between an amount Rs of birefringence of the STN liquid crystal cell and a retardation value Rc of the twisted retardation film ranges from 0.15 xcexcm to 0.25 xcexcm.
This sufficiently prevents coloring of a metallic display portion while securing a sufficient amount of reflected light of the white display in the background.
The above and other objects, features and advantages of the invention will be apparent from the following detailed description which is to be read in conjunction with the accompanying drawings.